This invention relates to a superconductor, and more particularly to an internal cooling type superconductor which is suitable for use in large-sized, high-magnetic field superconducting coils.
For superconducting coils large in size and capable of generating high magnetic fields, the use of an internal cooling type superconductor has been studied in view of, principally, problems on the cooling of the coils. Whether it is an immersion cooling type superconductor or the internal cooling type superconductor, a large-size superconductor which can conduct a high current in a high magnetic field is required to have a conductor strength which is enough to withstand high electromagnetic stresses and a stability owing to which even when the superconductor has been locally brought into a normal conducting state by a thermal or mechanical disturbance, it can return to a superconducting state again upon removing the disturbance. With regard to a coolant passage within the superconductor, regarding particularly the structure of the coolant passage, various conductor structures as shown in IEEE TRANSACTIONS ON MAGNETICS VOL. MAG-15, No. 1, JANUARY 1979 Pages 789.about.791, Japanese Laid-open patent application No. 54-57994 (1979), etc. have heretofore been proposed. However, they have demerits with merits and are unsatisfactory as the conductors for use in the large-sized high-field superconducting coils.
In FIG. 1 of the former document, there is shown a superconductor in which a large number of superconducting Nb.sub.3 Sn strands are formed into a cable and wrapped in a stainless steel jacket. The cooling is provided by forcing supercritical helium at 4 K through the interstices of the cable. The superconductor has the advantage that both the section of the coolant passage and the circumferential length of the passage are great, but has the disadvantages that the section of the divided coolant passage is non-uniform, so an elongated conductor has the possibility that the coolant passage will be locally closed up, and that the composite superconducting wire is moved by an electromagnetic force; the resultant heat generation renders the superconductor unstable.
In the latter document, for example, in its FIG. 6, there is disclosed a superconductor in which a pair of plates each including Nb.sub.3 Sn composite superconducting material are arranged in parallel with a spacing there between, and in the spacing a waved plate is disposed whereby a plurality of coolant passages are defined. By this cooling construction, it does not seem that a high stability can be obtained, which will become apparent from the explanation of the invention.